Parahydroxybenzoic acid has been used for manufacturing a wide variety of products including liquid crystalline polymers and preservatives for cosmetics and medicines. Generally, parahydroxybenzoic acid is prepared by a method in which phenol is reacted with potassium hydroxide to give potassium phenoxide, and the obtained potassium phenoxide is reacted with carbon dioxide under pressure to give potassium parahydroxybenzoate and then, parahydroxybenzoic acid is isolated by means of aciding out procedure, i.e. by adding a mineral acid to the salt.
For a long time, the Kolbe-Schmitt reaction, a solid-gas phase reaction, had been employed for the reaction between potassium phenoxide and carbon dioxide. This reaction, however, has several problems such as the long reaction time, the high amount waste of the starting materials in the side reaction, and the difficulty of attaining a stable yield. In order to solve those problems, a number of methods have been proposed.
One of the inventors has proposed a process for preparing parahydroxybenzoic acid, which comprises the step of reacting potassium phenoxide with carbon dioxide in the presence of an appropriate solvent in a suspension at a temperature equal to or more than 180° C., wherein phenol in an amount required for reacting with dipotassium parahydroxybenzoate to give potassium phenoxide is added to the reaction before the initiation of the carboxylation step. The process can produce potassium parahydroxybenzoate in a short time with high yield in a continuous manner (Japanese Patent Publication (KOKOKU) No. 9529/1970).
Parahydroxybenzoic acid can be obtained by precipitating the resulting potassium parahydroxybenzoate with acid. Thus obtained parahydroxybenzoic acid is then isolated from the solution by filtration with the aid of centrifugation, washed with water, and dried to be used as a starting material for manufacturing, for example, liquid crystalline polymers.
Parahydroxybenzoic acid, however, is highly soluble in aqueous solvent and therefore the yield of the product is low when the precipitated product is collected from aqueous solvent, e.g., by precipitation with acid. For this reason, to improve the yield, the solution containing parahydroxybenzoic acid is usually cooled to around room temperature to give slurry and then centrifuged to collect parahydroxybenzoic acid. The resulting product is crystalline parahydroxybenzoic acid monohydrate containing a lot of water which comes from the hydrated water and the aqueous solvent. It brings the necessity of a lot of energy to eliminate the water in the drying step, resulting in the high production cost and the long processing time.
In addition, the size of the obtained crystal is small. When thus obtained crystals are added into a reaction vessel as a starting material for manufacturing liquid crystalline polymers or the like, fine particles of parahydroxybenzoic acid fly in the air as powder dust, and t therefore they are difficult to handle.